This invention relates to a non-invasive cardiac output monitor; more particularly, it relates to a device for long-term or continuous use in such monitoring.
The continuous monitoring of aortic blood velocity and derived parameters by Doppler ultrasound or monitoring by a combination of ECG and Doppler ultrasound have not previously been done.
In simple terms, non-invasive monitoring of cardiac output means determination of the quantity of blood pumped by the heart, without entering the body. The use of ultrasound equipment has been proposed for this purpose, but such is attended by significant problems. Unless the user has a high level of skill, the results are unreliable since there is no means for the equipment to distinguish between systolic, i.e. wanted, blood flow and diastolic, i.e. unwanted blood flow. Additionally, there is no means for the system to assess signal-to-noise ratio of the flow signals, a function which is necessary since poor signal-to-noise ratio causes erroneous measurements. Furthermore, for meaningful measurement, the transducer must be accurately positioned and must be held in place in such a way that appropriate pressure is applied. Attempts to position the transducer using, for example, adhesive tape are, at best, only partially successful. Moreover, particularly for longterm use, the positioning means must not cause unnecessary discomfort to the patient and must allow body movement without disturbing the monitoring.
The present invention satisfies requirements for distinguishing between systolic and diastolic flow, assessing signal-to-noise ratio and transducer positioning for long term use, and offers significant advantages to a clinician.
In general terms, a cardiac output monitor based on the ultrasound technique will typically comprise the following units; a transducer, containing the ultrasound transmitter/receiver elements, which is manually held in place in the patient's suprasternal notch. The transducer is connected to the main instrument by a cable. The instrument contains a Doppler front end system which generates and receives the ultrasound signals, to and from the transducer, respectively, and which provides an audio output of the Doppler-shifted ultrasound. This audio signal is then passed to a spectrum analyzer or zero crossing counter which extracts, in real time, the maximum frequency present containing significant energy. The maximum frequency present during systole represents aortic blood velocity, hence an integration of this function, for each beat, represents the distance travelled by the blood during each beat. This is commonly referred to as "stroke distance". Aortic cross-sectional area is then determined, either by direct measurement, or by the use of a nomogram. Stroke distance multiplied by aortic cross-sectional area, multiplied by pulse rate then equals cardiac output, a calculation typically performed in software. The thus-obtained cardiac output value may then be displayed on LED's or LCD's. In use, the transducer is manually positioned in the suprasternal notch and manipulated until some predetermined criterion is maximised, for example the pitch of sound heard via the loudspeaker, the cardiac output value displayed or the peak frequency as displayed on a bar-graph. The thus-obtained cardiac output value is noted and the transducer is removed. In general, such instruments have not been well accepted by medical staff. One of the main problems is that they tend to be unreliable, that is, the instruments are susceptible to the interfering effects of other flows and movements of patient organs relative to the transducer. Erroneous results may occur, without the user being aware. Another major problem is that, as in the invasive techniques that the ultrasound methods seeks to replace, measurements are intermittent only. A continuous method of monitoring cardiac output is far more desirable since it enables monitoring throughout therapeutic manoeuvres and exercise tests, for example.
Surprisingly, it has now been possible to design an advantageous ultrasound sensor and instrument which overcomes these problems to a significant degree.
The present invention provides a device for positioning/retaining a transducer relative to a body characterised in that it comprises a supporting member for placement on the body, connected thereto one end of a flexible and elongate arm, which has a mount for the transducer at or near the remote end thereof, and means for locking the arm in a fixed position relative to the body.
Generally, the arm comprises a plurality of interengaging segments threaded on a wire. The tensioning wire may be of solid construction, but, if woven as is preferable, the cable is such that axial twisting on changing tension is minimised. The segments may be locked in position by a tensioning means, which may be of the screw and lever--type or the cam and lever--type. The same or different material may be used for the segments, which either all have one generally convex face and an opposing generally concave face or alternate between those having both faces generally concave and those having both faces generally convex.
The present invention also provides an apparatus for monitoring aortic blood velocity and derived parameters by Doppler ultrasound and/optionally, ECG comprising the present device and further provides that use of such an apparatus.
According to the present invention, there is provided a sensor device which is particularly suitable for use in non-invasive monitoring of cardiac output. In general terms, the present device is a positioning/retaining means for a transducer, for example a conventional ultrasound transducer, on the body surface, in particular making firm contact in the suprasternal notch. In accordance with the present invention, a transducer is mounted at one end of a flexible/fixable arm, a so-called `lockable snake`, and may be conventionally connected to the Doppler front end inside the instrument. The "lockable snake" comprises threaded segments each possessing a concave face and a convex face in a preferred embodiment, mating with adjacent segments. The segments are commonly made of glass loaded polycarbonate. Generally, such segments are threaded on a suitable wire. At the end of the `lockable snake` remote from the transducer is a tensioning means. Until tension is applied to the wire, the segments may move relative to one another, resulting in a flexible `snake`, which allows positioning as desired. Once the transducer is located as required, tension may be applied to the wire, thus `engaging` the segments and holding the configuration. Tension may be applied by various known means, commonly relying on screw and lever and cam adjustment. The tensioning means end of the snake is mounted on a supporting member. For example, a three-legged plastic supporting member may be used, which may be fixed to a patient's chest so that the transducer may be positioned as required. The supporting member may be fixed to the chest in various ways. It may be stuck in place with adhesive pads or held in place by an elasticated strap or straps or by a combination of such means, for example. A particularly convenient combination is to use standard ECG electrodes as a method of adhesion, together with a single neck strap. This provides the ECG signals needed by the main instrument from the same sensor as provides the Doppler signals. Otherwise, the ECG signal may be obtained by a standard, separate ECG lead.